In connection with the monitoring of a desired area, it is frequently important to be able to accurately detect intruders and other objects with a relatively high degree of resolution. Two-dimensional systems that include video cameras, for example, have been commonly used for monitoring and intrusion detection purposes. However, such systems have certain deficiencies including the inability to see at night and the inability to immediately distinguish an intrusion from safe or normal conditions. Such systems also cannot provide information concerning the distance an object is located from a reference location.
One known system has been proposed for overcoming such deficiencies. In the publication entitled "Three-Dimensional Vision System for the Adaptive Suspension Vehicle" of Zuk et al. published January, 1983 and sponsored by Defense Advance Research Projects Agency of the Department of Defense, a three-dimensional imaging system is described. This system is intended to provide a three-dimensional image of a scanned area using light outputted by a laser source. Scanning horizontally is achieved using a multi-sided or polygon mirror and a folding mirror. Scanning in the vertical direction is accomplished by means of a nodding mirror. Even though this scanning mechanism may function adequately in an experimental or testing environment, its large and relatively cumbersome size renders it unacceptable for certain, desired applications.
With regard to other previously devised scanning techniques, it is well-known to utilize counter-rotating optical wedges or prisms in order to obtain a horizontal scan of a scene that is being viewed. In U.S. Pat. No. 1,647,631 to Ives entitled "Optical System", an optical system is disclosed in which two sets of optical wedges are employed. One set of counter-rotating wedges produces a scan in a horizontal direction while the second set of counter-rotating wedges produces a scan in a vertical direction. Similar in significant respects to the apparatus disclosed in the Ives patent, is the optical system disclosed in U.S. Pat. No. 3,226,721 to Gould and entitled "Scanning Antenna Utilizing Four Rotary Prisms to Produce Rectilinear Scan and Fifth Prism to Produce Conical Scan." Two sets of pairs of counter-rotating wedges are utilized. Such wedges are rotated by means of a gear assembly driven by a motor. The scanning mechanisms disclosed in these two patents do not describe the hardware that is employed for receiving the light energy returned from the scanned area. In another known optical system that does not utilize scanning techniques, a system is disclosed for comparing the position of an object with that of a reference object. In U.S. Pat. No. 3,552,859 to Snyder III and entitled "Optical Instrument for Determining the Parallelism or Nonparallelism of Two Reflecting Surfaces," a system is illustrated that uses a pair of interconnected dual beam modulators, each of which transmits and receives light signals or information for comparison purposes.
Such prior art systems, however, have failed to satisfy the competing needs that arise in the development of a three-dimensional imaging system. Unlike other known systems, the present invention provides a relatively compact and efficient transmitter/receiver unit for rapidly raster scanning changing scenes with considerable accuracy and resolution.